The present invention relates to a biaxially stretch-oriented packaging film, in particular to a tubular artificial sausage casing, which comprises (a) a linear aliphatic polyamide, (b) a polyterephthalic acid ester, (c) an aromatic polyamide comprising a branched aliphatic diamine and an aromatic dicarboxylic acid or an aromatic diamine and an aliphatic dicarboxylic acid, and (d) a dye.
Sausage casings formed from a polymer mixture comprising the above components (a) and (b), and which may contain a dye, are known. Such mixtures are disclosed in EP-A-0,176,980, corresponding to U.S. Pat. No. 4,659,599. However, the strength and expansion properties of the casing are inadequate.
It is known that for the production of dyed plastic films, the dye is added as a masterbatch to the granules of plastic material prior to extrusion, whereby dye concentrations of between 1 and 10%, relative to the amount of plastics, are required to achieve intensive coloring. When dyed tubings are produced from the known polymer mixture of polyamide and polyester, serious difficulties arise on addition of certain dyes in quantities of more than about 1%, relative to the total quantity. The unstretched film obtained by extrusion contains lumps which are due to non-uniform distribution of the color pigments. This results in the formation of holes in the film. Quite frequently, the unstretched film cannot be subjected to subsequent stretching. Even when high forces are applied, stretching is impossible, and the film has a tendency to burst.
Attempts to solve these problems have been made either by employing relatively small amounts of dye resulting in insufficient coloring of the film, or by employing other dyes. The latter alternative is unsatisfactory because of the limited number of suitable dyes. A great number of dyes are not permitted in food packaging materials or should not be used for reasons of environmental safety. Other dyes are thermally unstable and decompose during the extrusion of the plastic melt. A solution to these difficulties has consisted in not dyeing the polymer material at all prior to extrusion, but providing a continuous colored print on the finished film.
DE-C-27 16 710 describes films of polyamide mixtures which are biaxially stretched in stages. These films comprise a linear aliphatic polyamide as the main component and polyamides containing cyclical members as an optional further component. The last-mentioned, ring-containing polyamides are copolymers comprising three different structural units. Examples of such structures are units of aromatic diamines, in particular xylylene diamine groups, or of aromatic dicarboxylic acids, e.g., terephthalic acid.
EP-A-0,065,278 discloses a film comprised of a polyamide mixture, wherein one component is a partly aromatic polyamide and the other component is a linear aliphatic polyamide, such as polyamide-6 or polyamide-6,6. In the partly aromatic polyamide, either the diamine units or the dicarboxylic acid units have an aromatic structure. An example mentioned in the publication is a condensation product from 2,2,4- and 2,4,4-trimethyl-hexamethylene diamine and terephthalic acid. None of these publications, however, propose a solution to the aforementioned problems.